1. Technical Field
This invention relates to the field of medical imaging, and specifically to a medical imaging apparatus that enables exploitation of the motion of an imaging detector to acquire differing views of the object to be imaged.
2. Description of the Background Art
Nuclear medicine imaging, for example PET or SPECT, uses radiation to acquire images that can show both the anatomy and the function of organs or tissues in a patient's body or other object of interest. In nuclear medicine imaging, radiopharmaceuticals are introduced into the body. These radiopharmaceuticals are attracted to specific organs or tissues and produce gamma emissions which leave the body or object. One or more detectors are positioned or move around the patient or object to be imaged to detect the gamma radiation emitted from the patient's body. This information is processed by computer to calculate the point of origin of the gamma radiation. Accumulation of a large number of gamma positions allows the instrument to display an image of the object under study.
In PET imaging, two 511 keV gamma rays are simultaneously produced upon decay or annihilation of a positron and travel in opposite directions. Scintillation detectors on opposite sides of the object being imaged produce an electrical pulse when the gamma ray interacts with a scintillation crystal. When the two detectors simultaneously produce an electrical pulse on opposite sides of the object, detecting the decay of a positron, the line connecting the positions where the gamma ray was detected is assumed to pass through the point where it originated. In single photon imaging, a collimator is placed in front of the scintillation crystal detector. The collimator allows only gamma rays aligned with the holes in the collimator to pass through to the detector. Thus, the line of origin of the gamma radiation is inferred from the alignment of the collimator and the detector.
Many nuclear medicine imaging systems are designed with detectors that move about the object being imaged. For example, nuclear medicine gamma cameras (detectors) perform single photon emission computed tomography (SPECT), a procedure in which an object to be imaged traditionally is placed on a rigid, horizontal imaging platform or imaging bed while one or more gamma cameras orbit the object, acquiring multiple images from different views. The images captured from different angles are reconstructed mathematically to provide a three-dimensional image of the object. In the majority of these prior art apparatuses, the gamma cameras are physically attached to an annular device which rotates, moving the detectors in a circular orbit around a stationary axis of rotation and the separate, stationary imaging platform. In this way, the detectors can capture projection views of the object from many different angles.
In known imaging systems, the object being imaged and the platform on which it is supported during imaging is not attached in any way to the imaging detector system. This configuration results in several engineering challenges. The angle of the detector is coupled to an imaginary reference frame in space. In SPECT and other imaging methods, image quality degrades with increasing distances between the detector and the object being imaged. Therefore, this distance should be minimized. However, orbital motion combined with minimization of distance from the object creates a potential for collision of the detector with the object being imaged, which could damage the delicate instrumentation of the detector and injure a patient being imaged. To ensure safety of the imaging apparatus, it is thus necessary to include a collision detection mechanism or other means to prevent detector-object collision.
Additional challenges in systems which require the detector to orbit a stationary axis of rotation are the need for maintaining the object within the same field of view of the camera or detector during its orbital motion while keeping the distance between the object and the detector to a minimum for each projection view. The design of currently known systems, constructed with the detector systems mounted onto an annular device which limits the motion to a defined orbital path, allows the detectors the necessary range of angular projections and defines a repeatable detector pathway that can be covered by a protective barrier to avoid collisions. The distance between the detector and the object being imaged, however, is not minimized, resulting in less than maximal image quality.
Prior systems also have provided radial motion to the detector systems, which allows the angle of the detector relative to the imaging platform to be adjusted so that the distance to the object being imaged can be minimized but remain constant during the imaging process. For this type of apparatus, the stationary position of the imaging platform and the orbital motion of the detector must be calibrated, a “center of rotation” calibration. This calibration must be performed periodically as the apparatus is used and parts for setting the radial position of the detectors become worn. Other prior systems have instead employed multiple non-rotating but adjustable detectors to provide simultaneous, parallel acquisition of different projection angles.
Thus, currently available medical imaging apparatuses, for example apparatuses for SPECT, acquire projection angle images from a number of angles either by movement of the expensive and fragile but heavy and cumbersome detectors to gain different angular perspectives of the object to be imaged, or using a sufficient number of detectors positioned about the object such that movement is not necessary. Precise and controlled movement of the detector systems is difficult due to their size, mass and fragility. The need to bring the detectors as close to the object as feasible in order to optimize image quality requires that the detectors move radially, however this radial freedom of motion creates the potential for object-detector collisions and the potential for uncertainty to the position of the detector with respect to the object as the detector is brought to different angular positions.
There is a need in the art for medical imaging apparatuses which overcome the disadvantages of the currently available systems, in particular the disadvantages created by a system in which detectors are required move to different angles in orbit around a stationary object.